Amoebiasis

Amoebiasis is an infection caused by the protozoan, Entamoeba histolytica, which is usually contracted by drinking water or eating food contaminated with amoebic cysts. Most of the infected people are asymptomatic but the disease has the potential to be chronic and it is estimated by WHO that about 70,000 people die annually worldwide.

Symptoms that appear within 2-4 weeks of infection, can sometimes last for years, which can range from mild diarrhoea to dysentery with blood and mucus. The blood comes from the damaged lining of the intestine. In about 10% of invasive cases the amoebae enter the bloodstream and may travel to other organs in the body, such as liver where blood from the intestine reaches first but they can end up almost anywhere.

In asymptomatic infections the amoeba lives by eating and digesting bacteria and food particles in the gut because it does not come in contact with the intestine due to the protective mucus layer lining the gut. Disease occurs when amoeba comes in contact with the intestinal lining, when it secretes enzymes that destroy cell membranes and proteins resulting in flask-shaped ulcers in the intestine. Entamoeba histolytica ingests the destroyed cells by phagocytosis and is often seen with red blood cells inside. A granulomatous mass known as an amoeboma may form in the wall of the colon due to persistant cellular response, which may be confused with cancer.

Symptom of Amoebiasis

Gastroenteritis, diarrhea or dysentery with abdominal pain and exhaustion is the main symptom of amoebiasis. Poor appetite or fear of food due to abdominal bloating and cramps and loose stools can occur. Later, with increased intensity of infection, fever, nausea and bloody stools with slimy mucous occurs and complicates the condition. In due course, the patient loses weight and stamina. Sometimes allergic reactions can occur throughout the body due to the release of toxic substances or dead parasites inside the intestines. Dehydration and gas formation and foul-smelling stools commonly occurs and diarrhea comes and goes. Mucus and blood appears in the stool.

Diagnosis

Asymptomatic human infections are usually diagnosed by finding cysts shed with the stool. Various flotation or sedimentation procedures have been developed to recover the cysts from fecal matter and stains help to visualize the isolated cysts for microscopic examination. Since cysts are not shed constantly, a minimum of 3 stools should be examined. In symptomatic infections, the motile form (the trophozoite) can often be seen in fresh feces.

Serological tests exist and most individuals (whether with symptoms or not) will test positive for the presence of antibodies. The levels of antibody are much higher in individuals with liver abscesses. Serology only becomes positive about two weeks after infection. More recent developments include a kit that detects the presence of ameba proteins in the feces and another that detects ameba DNA in feces. These tests are not in widespread use due to their expense.

Colon biopsy is still by far the most widespread method of diagnosis of amoebic dysenteri around the world. However it is not as sensitive or accurate in diagnosis as the other tests available. It is important to distinguish the E. histolytica cyst from the cysts of nonpathogenic intestinal protozoa such as Entamoeba coli by its appearance. E. histolytica cysts have a maximum of four nuclei, while the commensal Entamoeba coli has up to 8 nuclei.

Additionally, in E. histolytica, the endosome is centrally located in the nucleus, while it is off-center in Entamoeba coli. Finally, chromatoidal bodies in E.histolytica are rounded, while they are jagged in Entamoeba coli. However, other species, Entamoeba dispar and E. moshkovskii, are also a commensal and cannot be distinguished from E. histolytica under the microscope. As E. dispar is much more common than E.histolytica in most parts of the world this means that there is a lot of incorrect diagnosis of E. histolytica infection taking place. The WHO recommends that infections diagnosed by microscopy alone should not be treated if they are asymptomatic and there is no other reason to suspect that the infection is actually E. histolytica.

Prevention

To help prevent the spread of amoebiasis around the home:

Wash hands thoroughly with soap and hot running water for at least 10 seconds after using the toilet or changing a baby’s diaper, and before handling food

Avoid raw vegetables when in endemic areas as they may have been fertilized using human feces.

Boil water or treat with iodine tablets.

Treatment

E. histolytica infections occur in both the intestine and (in people with symptoms) in tissue of the intestine and/or liver. As a result two different sorts of drugs are needed to rid the body of the infection, one for each location. Metronidazole, or related drugs such a tinidazole or ornidazole are used to destroy amebae that have invaded tissue. It is rapidly absorbed into the bloodstream and transported to the site of infection. Because it is rapidly absorbed there is almost none remaining in the intestine.

Since most of the amoebae remain in the intestine when tissue invasion occurs, it is important to get rid of those also or the patient will be at risk of developing another case of invasive disease. Several drugs are available for treating intestinal infections, the most effective of which has been shown to be Paromomycin (also known as Humatin); Diloxanide furoate is used in the US. Both types of drug must be used to treat infections, with metronidazole usually being given first, followed by paromomycin or diloxanide. E. dispar does not require treatment, but many laboratories (even in the developed world) do not have the facilities to distinguish this from E. histolytica.

For amoebic dysentery a multi-prong approach must be used, starting with one of :

Metronidazole, 500-750 mg, three times a day for 5-10 days.

Tinidazole, 2g once a day for 3 days is an alternative to metronidazole.

Ornidazole, 500 mg, twice a day for 5 days.

In addition to the above, one of the following luminal amebicides should be prescribed as an adjunctive treatment, either concurrently or sequentially, to destroy E. histolytica in the colon:

Paromomycin, 500mg three times a day for 10 days.

DiloxanideFuroate, 500mg three times a day for 10 days.

Iodoquinol, 650mg three times a day for 20 days.

For amoebic liver abscesses the following drugs are prescribed:

Metronidazole, 400mg three times a day for 10 days.

Tinidazole, 2g once a day for 6 days is an alternative to metronidazole.

Diloxanidefuroate, 500mg three times a day for 10 days must always be given afterwards.

Doses for children are calculated on the basis of body weight and a pharmacist should be consulted for help.